Electric DC Motors

Electric DC motors and in particular their lightweight versions are most common propulsion systems in UAVs.

There are two classes of electric motors:

▪ brushed,

▪ brushless.

Electric DC motors vary in diameter from a couple of mm to 15cm with a power consumption of some mA to 200A.

10.4.2.2.1. Brushed DC motors

Brushed motors use an internal switching system to the alternate current direction, thus changing magnetic field. It is pretty easy to recognize the brushed motor as it has just two wires (brushless has three). Speed can be controlled via control of the energy delivered, i.e. changing voltage (directly or rather via PWM duty cycle). Brushed, coreless motors are designed to rotate in one direction. This is the reason why brushed motors are marked CW (Clock-wise) and CCW (Counter-wise). While some of them can operate other direction, it is not very efficient. Because brushed motors use the brushed switch inside, named commutator, that uses friction, it wears out over time thus brushed DC motors popular only in smallest, miniature drones. Brushed motor construction is not scalable in terms above some diameter; weight to torque ratio is rapidly decreasing. Because of the mechanical, friction-based, commutator construction, brushed DC motors used to be considered less reliable than brushless ones. The advantage is simplicity on powering and speed control, usually using a single MOS-FET transistor and PWM.

Miniature DC brushed motors are marked pretty frequently with their external sizes: i.e. 8520 means 8.5mm diameter, 2cm length. A common maximum voltage is 1s (up to 4.2V) on most of the miniature drone brushed DC motors. While some report motors can operate on higher voltage (even 2S that is equal to 8.4V max), they tend to overheat then and break quickly. Sample brushed motor is present in Figure 98.

Figure 98. Brushed DC motors (pair, CW and CCW) for miniature quadcopter

Note, CW and CCW motors are distinguishable with cable colours: CW is usually red+blue while CCW is white+black (as in Figure 98). There is no official standard, however.

There is a class of brushed motors for UGVs that are much bigger and support higher voltage, but we do not consider them in drone's section.

10.4.2.2.2. Brushless motors

Brushless motors used to be designed for not so small drones, as their internal construction is pretty complex. Recently, however, brushless motors range was extended with miniature, and super-miniature motors along with assembling technology development and they tend to replace brushed motors even in miniature UAVs. Still, they are more expensive comparing to brushed motors and require complex control electronics (ESC, Electronic Speed Controller). Brushless motors can operate in both directions. Brushless motors connect with 3 cables to the ESC.

Changing rotation direction is as simple as swapping two of three wires (any pair).

Some of the medium size brushless motors are marked as CW and CCW. This is not because of their inability to rotate, but rather to prevent self-loosening of the propeller mounted.

Brushless motors do not contain commutator that wears out over time: they are more reliable and lasting longer than brushed ones.

Brushless motor is composed of the stator with coils, connected permanently to the wired terminals and rotating rotor with permanent magnets (Figure 99).

Figure 99: Sample brushless, outrunner motor interior

Universal (non-proprietary) motors have usually marked the way one can read its features, i.e.

HK-4015-1450KV means the motor is:

▪ HK - manufacturer marking (here HobbyKing),

▪ 40 - motor diameter,

▪ 15 - motor height,

▪ 1450KV - 1450 rotations per 1 Volt (see remark below).

Motor's electrical features are defined via maximum voltage it can handle, maximum current and most of all, its rotation speed given as the number of revolutions per 1 Volt of power. Increasing voltage (within maximum limit given) speeds up the motor. This is usually rated under no-load (without propeller) so may differ in real-world scenarios. A rule of thumb is higher KV, faster the motor rotates. It does not necessarily mean it is more energy-efficient, as faster rotation is usually for smaller motors; thus, it delivers lower torque.

Motor's minimum and maximum voltage is usually provided not within the Volt but rather as a number of “S”. Meaning of 1S is explained better in drone's power section, but here please assume that 1S may vary between 3.3V (minimum) and 4.2V (maximum), 3.7V on average.

Some motors also contain a note on its internal, electromagnetic construction, i.e. 12N14P means:

▪ 12N number of permanent magnets in the stator,

▪ 14P number of coils in the rotor.

In general, the lower the N and P are, the more powerful engine is, but on the other hand, higher N and P means smoother and more precise rotation (i.e. necessary for gimbals). Typical for multirotor is 12N14P.

The number of permanent magnets in the stator is always multiplied of 3 because the three-phase controller controls it.

The number of coils has to be different than the number of permanent magnets!

Motor's windings (cable diameter and wiring) has a direct impact on its resistance, thus on motor's KV. In short, the thicker and shorter the cable (fewer turns), the lower resistance, the more KV the motor has.

The winding (wiring) can be single strained and multi strained (wired using single or parallel cables, where the parallel is usually three).

Single strained wiring tends to have better heat management thus are used for higher voltage, i.e. 5-6S. Because of the bigger diameter, you cannot pack it very well; thus single strained motors are bigger than multi-strained ones.

Multi strained wiring can be better packed because of smaller empty spaces between wires; thus such coil creates a higher magnetic field than single strained wiring, that means multi strained motors are more energy-efficient and smaller.

Wiring construction neatness is important here because messy wiring disturbs magnetic field generation and lowers overall performance.

Physical properties of the motor include also:

▪ Maximum thrust (eventually a list of thrust generated, regarding voltage and propeller size/

type).

▪ Weight.

There are types of mechanical constructions:

▪ Inrunner: The external body (can like) is static while the rotor is inside of the motor. There is a shaft and construction mimics brushed motors. In such construction, the rotor is called the core.

▪ Outrunner: The stator with coils is inside while the rotor with magnets is outside: most of the engine is rotating, including external housing. In such construction, the rotor is called the motor bell.

Each construction has some features comparing inrunner to outrunner. In particular, the following is to consider when juxtaposing features of comparable two:

▪ Inrunner has a smaller diameter than outrunner;

▪ Outrunner has a lower profile (height) than inrunner;

▪ Outrunner body rotates;

▪ Inrunner has better heat dispersion (coils are located outside, magnets inside) than outrunner (this is partially true, cause modern outrunner's shell is constructed like a kind of fan, to ventilate interior);

▪ Outrunner generates larger torque than inrunner;

▪ Inrunner has higher KV (rotations per volt) than outrunner;

▪ Inrunner has better energy efficiency.

In the table below, there are proposed applications with respect to the inrunner and outrunner motors (Table 3).

Table 3. Inrunner and Outrunner applications

Inrunner Outrunner racer fixed-wing, EDF, RC Car, RC

Boat multirotor, helicopter, RC airplane (for 3D evolution), gimbal

In the Table 4 there is a juxtaposition of a UAV quadcopter frames and corresponding motors and propellers. One may use them as a starting point when planning new construction.

Table 4. A proposal for quadcopter frame size, corresponding propeller and motor Frame Size Propeller Size Motor Diameter KV Range

?15cm ?3in 11xx↔13xx >=3000KV

18cm 4in 18xx↔2204 2600↔3000KV

21cm 5in 2205↔2306 2300↔2600KV

25cm 6in (2206-2208)↔2306 or taller 2000↔2300KV

35cm 7in 25xx 1200↔1600KV

45cm 8-11in 26xx and larger 800↔1200KV

90-100cm 13-20in 4114↔6010 320↔450KV

Please note, it is very individual to construct a drone, so above values are on average.